In an audio communication system with several endpoints (or clients), there is typically a recurring need to provide an audio signal being an additive mix of live signals originating from different endpoints and approximating the sound that would have been heard if all the communicating parties had been present in one location. This is generally desirable in voice conference systems and video conference systems. The literature contains descriptions, discussions and solutions to many problems arising in connection with such mixing, including reformatting, delay limiting, load reduction, synchronization, quality of service issues and the like.
The present invention is applicable particularly to communication systems with non-uniform sampling rates. An important example is systems where individual communication endpoints are allowed to encode and/or decode audio signals at a selectable sampling rate, such as 8 kHz (narrowband, as per ITU G.711), 16 kHz (wideband) and 32 kHz (super wideband). At critical sampling, this means that the spectral range, in which the spectral content is defined and encoded, is allowed to vary between different audio signals in the audio communication system. Because the sampling rate is allowed to vary, a system entity (e.g., conference server) that is responsible for generating the mix signal may be receiving input audio signals at different sample rates.
A straightforward though computationally costly way of enabling the mixing as such may be to decode the input audio signals before mixing and then re-encode the mix signal afterwards. Again, however, the endpoints may require different coding formats with different sampling rates, and so the re-encoding process may need to be repeated once for each unique format and sampling rate. Alternatively, the mixer outputs a basic signal which is then reformatted into whatever formats are required by the connected endpoints. Either way, this places considerable load on the server.
All the figures are schematic and generally only show parts which are necessary in order to elucidate the invention, whereas other parts may be omitted or merely suggested. Unless otherwise indicated, like reference numerals refer to like parts in different figures.